Optical relative motion detection devices typically utilize image correlation techniques to determine relative motion between the navigation device and a surface by capturing images of the surface as the navigation device passes over the surface or as the surface moves past the navigation device. Both the displacement and the direction of the relative motion of the navigation device with respect to the surface are determined by comparing one image with the following image. Typically, intensity variations due to shadows cast on the surface are detected and the sensitivity and applicability of this technique depend on the intensity contrast in the captured images. Relative motion navigation devices are used, for example, for computer screen pointer (e.g. mouse) control.
U.S. Pat. Nos. 5,786,804, 5,578,813, 5,644,139, 6,442,725, 6,281,882 and 6,433,780 disclose examples of optical mice, other hand-held navigation devices and hand-held scanners. These patents are incorporated herein by reference.
Typical existing optical navigation devices use light emitting diodes (LEDs) to obliquely illuminate the surface to be navigated. Height variations on the surface, on the order of 5 to 500 μm, cast shadows described by geometrical ray optics. The size and contrast of the shadow pattern images depends in part on the type of surface through the size of the height variation. Typically, the detector is positioned to receive the reflection in the surface normal direction and the angle between the surface and the incident light is typically selected to optimize the contrast of the shadow pattern images as is familiar from dark field imaging. Typical values for the angle of incidence are in the range from about 5 degrees to about 20 degrees.
Smooth surfaces such as whiteboard, high gloss paper, plastic or painted metal present functional challenges to typical current optical navigation devices. In general, smooth surfaces are those containing less mid spatial frequency and more high spatial frequency structures. To increase the signal level, high optical power is required for LED illumination resulting in typical current draws in excess of 30 mA.